The present disclosure relates generally to styrenic resin compositions and methods for preparing the same. More specifically, the present disclosure relates to styrenic resins having improved extensional viscosities.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Markets for certain products, such as articles made from polymeric materials, have benefitted from improved techniques for producing such materials. For example, improvements in techniques for polymerizing monovinylarene monomers to produce styrenic resins, such as improved yields, increased polymerization efficiencies, and/or improved properties of the styrenic resin, have led to styrenic resins being used in a wide variety of everyday applications. High impact polystyrene (HIPS) and crystal/general purpose polystyrene (GPPS), for instance, are styrenic resins that are presently used in a wide variety of products including medical devices, food packaging, food service articles, beverage containers, toys, appliances, jewel cases (e.g., for compact or digital video discs), and electronics.
In some GPPS applications, particularly food service, food packaging and large appliances, a sheet of a styrenic resin is extruded and subsequently thermoformed into a useful shape. In certain processes, the sheet may be a foamed sheet. An article (e.g., a disposable cup) may be cut out of the thermoformed shape. For economic considerations, the removed portion of thermoformed shape is typically reprocessed back into the extrusion process. The additional shear and heat history placed on the styrenic resin as a result of this reprocessing causes a certain degree of chain scission of the styrenic polymer chains. The reprocessed resin will therefore possess a lower molecular weight compared to the starting virgin resin. This lower molecular weight results in a variety of undesirable characteristics in the final part such as poor sidewall distribution in a thermoformed part; low sheet orientation and poor toughness in oriented film and sheet applications; and high open cell content, low orientation, and poor toughness in foam sheet structures and thermoformed articles.
The above characteristics are generally related to the extensional viscosity (EV) of a given styrenic resin. The EV of a resin is a measure of the viscosity coefficient of the resin when an extensional stress is applied. In other words, EV is the viscosity of stretching. Generally, as the EV of a resin increases, the undesirable characteristics mentioned above may be mitigated by enabling higher degrees of chain entanglement and chain orientation. Accordingly, it would be advantageous if the EV of styrenic resins could be increased to produce more consistent, better quality, and lighter weight parts.